1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus and in particular to a configuration of a side end regulating portion which regulates positions of side ends of sheets accommodated in a sheet cassette detachably mounted on the sheet feeding apparatus.
2. Description of the Related Art
Today, in widely-used image forming apparatuses including a copying machine, a printer, and a facsimile, sheets are fed to an image forming portion by a sheet feeding apparatus to form images. According to such a sheet feeding apparatus, a sheet cassette is usually detachably mounted on an apparatus main body and sheets accommodated in the sheet cassette are automatically fed to the image forming portion.
According to a sheet cassette used for such a sheet feeding apparatus, for example, a middle plate on which sheets are stacked and which presses the sheets against a feed roller is provided so as to be movable upward and downward. Further, in a sheet cassette with such a middle plate, a rear end regulating portion which regulates positions of side ends of sheets on upstream (hereinafter, rear ends) in a sheet feeding direction accommodated on the middle plate in a stacked manner is slidably provided so that sheets of different sizes can be accommodated. Further, a pair of side end regulating portions which regulates positions of side ends of sheets in a direction perpendicular to a sheet feeding direction (hereinafter, a width direction) is also provided.
The pair of side end regulating portions regulates the side end of a sheet and the rear end regulating member regulates the rear end of the sheet, so that the position of the sheet is always at a predetermined position. Consequently, when the sheet cassette is accommodated in the apparatus main body, sheets are stably fed regardless of a sheet size.
According to a sheet cassette with such a side end regulating portion, one of side end regulating portions is fixed and the other can slide in the width direction. The one fixed side end regulating portion (hereinafter, fixed side end regulating portion) serves as a reference surface in the width direction of a sheet. Sheets are fed along the fixed side end regulating portion, so that predetermined printing precision is accomplished.
According to such a sheet cassette, when a direction of mounting/detaching the sheet cassette is the width direction, at the time when the sheet cassette is set, sheets are sometimes shifted because of an inertial force to be separated from the fixed side end regulating portion. Conventionally, among a pair of side end regulating portions, a slidable side end regulating portion (hereinafter, movable side end regulating portion) is provided with a sheet pressing portion to which a force is applied by a biasing unit such as a spring for pressing sheets. When sheets are shifted, this sheet pressing portion presses the shifted sheets back to the fixed side end regulating portion.
External dimensions of sheets vary. For example, when the tolerance of a width direction dimension of a sheet is ±2 mm, the position where the sheet pressing portion protrudes must be in the range of ±2 mm at minimum with respect to the position of a width direction end of a normal sheet. Accordingly, the position of the movable side end regulating portion at the time of regulating the side end of a sheet must be determined in view of variations in external dimensions of sheets. When the position of the movable side end regulating portion is set as described above, when sheets are not stacked, the position where the sheet pressing portion protrudes enters the center of sheets at least by 2 mm.
To ensure a sheet regulating force by the side end regulating portion, a biasing force to be applied to the sheet pressing portion is sometimes increased. As the stacking amount of sheets is reduced, the rigidity of a sheet bundle is also reduced. Accordingly, when the biasing force to be applied to the sheet pressing portion is increased as mentioned above, as the stacking amount of sheets is reduced, the sheets are deformed, so that the sheets cannot be fed stably. When the biasing force to be applied to the sheet pressing portion is reduced so that the sheets are not deformed even if the stacking amount of sheets is reduced, a force of regulating positions of sheets when sheets are fully stacked is insufficient.
Devices for preventing deformation of sheets when fewer sheets are stacked while increasing a pressing force upon the fully stacked sheets are conventionally suggested. For example, a configuration which uses a link to positively vary the pressing force by the sheet pressing portion and reduces the pressing force when the stacking amount of sheets is reduced is suggested (see Japanese Patent Laid-Open No. 2000-118730). However, this configuration increases components, resulting in an increase in cost. A configuration that a relief is formed in a sheet pressing portion so that components are not increased and this relief reduces a pressing force by the sheet pressing portion or the relief does not contact the sheets so that the sheets are not pressed when the stacking amount of sheets is reduced is suggested (see Japanese Patent Laid-Open No. 2000-219330).
According to these conventional sheet feeding apparatuses and image forming apparatuses, a plurality of sheet pressing portions is sometimes provided on the movable side end regulating portion along the sheet feeding direction. As illustrated in FIGS. 8A, 8B, 9A, and 9B, sheets having different sheet feeding direction lengths are sometimes accommodated in a sheet cassette. FIGS. 8A and 8B illustrate a state that A4 size (297 mm×210 mm/ISO 216) sheets are accommodated in the sheet cassette. FIGS. 9A and 9B illustrate a state that A5 size (210 mm×148 mm/ISO 216) sheets are accommodated in the sheet cassette.
As illustrated in FIG. 8A, A4 size sheets Sa are stacked along a side regulating plate 714 serving as the fixed side end regulating portion that forms a reference surface which is a sheet conveyance reference in the width direction. A force is applied to the A4 size sheets Sa toward the side regulating plate 714 by sheet pressing portions 716 and 717 mounted on a side regulating plate 715 serving as an opposing movable side end regulating portion.
As illustrated in FIG. 8B, a biasing member 801 is placed between the side regulating plate 715 and the sheet pressing portion 716 and a biasing member 802 is placed between the side regulating plate 715 and the sheet pressing portion 717. These biasing members 801 and 802 enable the sheet pressing portions 716 and 717 to apply biasing forces P81 and P82 to the A4 size sheets Sa.
The biasing force P81 by the sheet pressing portion 716 on upstream in a sheet feeding direction is a biasing force required to be applied to the stacked sheets Sa with respect to the side regulating plate 714 so that the sheets are in a predetermined attitude, and is, e.g., 1.0 N to 3.0 N. The biasing force P82 by the sheet pressing portion 717 on downstream in a sheet feeding direction with respect to the sheet pressing portion 716 is applied to a fed sheet Sa at the time of sheet feeding to maintain the attitude of the sheet Sa to be in the predetermined attitude. This biasing force P82 is such that sheets are not deformed and is required to be 0.1 N to 0.5 N. The position where the biasing force P81 acts is preferably near the center of gravity of a sheet, i.e., a sheet feeding direction center of a side surface of the sheet (so the center, in the sheet feeding direction, of a side surface of the sheet) so that the sheet is not rotated about the sheet pressing portion 716 because of an impact when a sheet cassette is inserted.
The biasing force P81 by the sheet pressing portion 716 is applied to the A4 size sheet Sa. In this case, the attitude of the sheet Sa is determined by a triangular area connecting ends M81 and M82 of the side regulating plate 714 and a point of application of the biasing force P81. Assume that the sheet pressing portion 716 is the center of rotation of the sheet Sa and rotation of the sheet Sa is prevented by the side regulating plate 714.
As illustrated in FIG. 8B, when the area connecting the ends M81 and M82 of the side regulating plate 714 and the point of application of the biasing force P81 is an acute triangle, an effect that the side regulating plate 714 prevents the rotation of the sheet Sa with respect to a moment of rotation of the sheet Sa can be expected. The position where the biasing force P81 is applied by the sheet pressing portion 716 is generally in the range of ¼ to ½ of the length in the sheet feeding direction of the A4 size sheet Sa. Referring to FIG. 8B, the position where the biasing force P81 acts is set to ½ of the length in the sheet feeding direction of the A4 size sheet Sa, i.e., the sheet feeding direction center of the A4 size sheet Sa.
As illustrated in FIG. 9A, A5 size sheets Sb are stacked along the side regulating plate 714 like the A4 size sheets Sa. A force is applied to the A5 size sheets Sb toward the side regulating plate 714 by the sheet pressing portions 716 and 717. As illustrated in FIG. 9B, the sheet pressing portions 716 and 717 apply biasing forces P81 and P82 to the A5 size sheets Sb, respectively.
As described above, the position of the sheet pressing portion 716 is determined in view of the attitude and position of the A4 size sheet Sa. When the position of the sheet pressing portion 716 is set to such a position, as illustrated in FIG. 9B, the biasing force P81 is applied to a sheet feeding direction rear end of the A5 size sheet Sb and the A5 size sheet Sb is supported by an area from M83 to M84 with respect to the side regulating plate 714. M83 indicates a corner of the A5 size sheet Sb at a rear end in the sheet feeding direction. In this case, an area connecting M83, M84, and the point of application of the biasing force P81 is substantially a right-angled triangle.
Assume that the sheet pressing portion 716 is the center of rotation of the A5 size sheet Sb and the rotation of the sheet Sb is prevented by the side regulating plate 714. In this case, when the area connecting M83, M84, and the point of application of the biasing force P81 is a right-angled triangle, a unit which resists to a moment of counterclockwise rotation of the A5 size sheet Sb is not provided. Accordingly, the attitude of the sheet Sb cannot be maintained. As described above, because the biasing force P82 by the sheet pressing portion 717 is significantly reduced, the moment of rotation cannot be supported by the biasing force P2 by the sheet pressing portion 717.
When the stacking amount of the A5 size sheets Sb is reduced and thus the rigidity of a sheet bundle is reduced, the sheet feeding direction rear end of a sheet Sb is deformed by the biasing force P81 by the sheet pressing portion 716. Consequently, as illustrated by a line Sb′ in FIG. 9B, a front end of the sheet Sb is pressed out and the attitude of the sheet Sb cannot be maintained.
As described above, according to the conventional configuration, when the size (length in the sheet feeding direction) of accommodated sheets is different, positional shifts caused by an impact at the time of mounting/detaching a sheet cassette and deformation of sheets and changes in attitude of the sheets caused by a biasing force cannot be prevented for sheets of all corresponding sizes. For example, the sheet pressing portion can be provided for each sheet size. However, in this case, the configuration is complicated and an increase in cost is problematic.
The present invention has been achieved in view of the above problems, and provides a sheet feeding apparatus and an image forming apparatus that can prevent changes in attitude of sheets when a sheet cassette is mounted and deformation of the sheets and changes in attitude of the sheets after the sheet cassette is mounted regardless of a sheet size.